Experimental and Numerical Study of Immiscible Droplet Transport in Collapsible Vessels Under Altered Gravity Conditions

Understanding how altered gravity influences the transport dynamics of immiscible liquid drops in large, thin-walled vessels is critical for fluid systems in space physiology and diagnostics. Although droplet dynamics have been widely studied in rigid microfluidic channels, the behavior of buoyant droplets and deformable vessels under intermediate Re regimes and varying gravity remains poorly understood. This gap is relevant in microgravity where vessel flexibility and fluid-structure interactions create unique transport challenges. In this study, effects of gravity on droplet transport within a collapsible tube were explored using experimental and numerical methods. Experimentally, the influence of gravity was examined by varying the density ratio of immiscible fluids. The dispersed phase (Oil) and the continuous phase (water-glycerin mixture) were first selected to create neutrally buoyant drops. The composition of the continuous phase is changed to create density differences. High-speed cameras were used to capture droplet dynamics and its interactions with vessel walls. The effects of internal Weber (Weᵢ) number and external Capillary (Caₒ) number were evaluated using image analysis. These dimensionless parameters allowed the identification of flow regimes ranging from stable droplet translation to complex dripping or jetting behavior. In particular, low-Weᵢ, low-Caₒ conditions favored capillary dominated plug flow, while increased Caₒ led to asymmetrical drop profiles and helicoidal trajectories. The effect of gravity on the droplet motion in a hyperelastic vessel was investigated using a 3D Spectral Boundary Element Method, validated by experiments. It uses spectral points to discretize the surface and evaluate surface integrals with high accuracy and low computational cost. The outcomes reveal how gravity influences droplet shape, transport velocity and energy (pressure) losses. This study will serve as a foundation for future microgravity experiments, providing a framework for gravity sensitive fluid system design in space based biomedical and engineering platforms.